The retrovirus designated human immunodeficiency virus (HIV), particularly the strains known as HIV type-1 (HIV-1) and type-2 (HIV-2), have been etiologically linked to the immunosuppressive disease known as acquired immunodeficiency syndrome (AIDS). HIV seropositive individuals are initially asymptomatic but typically develop AIDS related complex (ARC) followed by AIDS. Affected individuals exhibit severe immunosuppression which makes them highly susceptible to debilitating and ultimately fatal opportunistic infections. Replication of HIV by a host cell requires integration of the viral genome into the host cell's DNA. Since HIV is a retrovirus, the HIV replication cycle requires transcription of the viral RNA genome into DNA via an enzyme known as reverse transcriptase (RT).
Reverse transcriptase has three known enzymatic functions: The enzyme acts as an RNA-dependent DNA polymerase, as a ribonuclease, and as a DNA-dependent DNA polymerase. In its role as an RNA-dependent DNA polymerase, RT transcribes a single-stranded DNA copy of the viral RNA. As a ribonuclease, RT destroys the original viral RNA and frees the DNA just produced from the original RNA. During the viral RNA-dependent polymerization process, RT's ribonuclease activity is required for removing RNA and leaving the polypurine tract preserved for initiation of DNA-dependent polymerization. As a DNA-dependent DNA polymerase, RT makes a second, complementary DNA strand using the first DNA strand as a template. The two strands form double-stranded DNA, which is integrated into the host cell's genome by HIV integrase.
It is known that compounds that inhibit enzymatic functions of HIV RT will inhibit HIV replication in infected cells. These compounds are useful in the treatment of HIV infection in humans. Classes of RT inhibitors include non-nucleoside active site competitive RT inhibitors (NNRTIs), such as efavirenz (EFV), nevirapine (NVP), etravirine (ETR), and rilpivirine (RPV), and active site RT inhibitors which include nucleoside reverse transcriptase inhibitors (NsRTIs) and nucleotide reverse transcriptase inhibitors (NtRTIs), collectively referred to as NRTIs. Examples of NsRTI's include 3′-azido-3′-deoxythymidine (AZT), 2′,3′-dideoxyinosine (ddI), 2′,3′-dideoxycytidine (ddC), 2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), 2′,3′-dideoxy-3′-thiacytidine (3TC), abacavir, emtricitabine and 4′-ethynyl-2-fluoro-2′-deoxyadenosine (EFdA) which is also known as a as nucleoside reverse transcriptase translocation inhibitor. Examples of NtRTIs include tenofovir (TFV, also known as PMPA, 9-(2-phosphonyl-methoxypropyl)adenine), tenofovir disoproxil fumarate (VIREAD®, U.S. Pat. Nos. 5,977,089, 5,935,946) and tenofovir alafenamide fumarate (U.S. Pat. Nos. 7,390,791, 8,754,065).
TFV belongs to a class of HIV anti-retroviral (ARV) agents known as nucleotide analog reverse transcriptase inhibitors (NRTIs). Tenofovir is a monophosphonate:

After being taken up by cells, TFV is first converted to tenofovir-monophosphate (TFV-MP) by adenosine monophosphate kinase and then to the active antiviral tenofovir-diphosphate (TFV-DP) by 5′-nucleoside diphosphate kinase.

TFV-DP inhibits HIV DNA synthesis by competing with the natural substrate, deoxyadenosine triphosphate, for incorporation into the complementary DNA strand by HIV reverse transcriptase; following incorporation, TFV acts as a chain terminator due to lack of a 3′-hydroxyl group that is required for addition of the next nucleotide. TFV has poor cellular permeability and thus has limited bioavailability. Tenofovir disoproxil fumarate (TDF) is approved for treating HIV infection and is marketed by Gilead under the trade name VIREAD™. The disoproxil prodrug improves cell permeability and absorption after oral dosing, with the pro-moiety being cleaved rapidly after absorption to yield the parent TFV. As a result, the circulating level of TFV is much higher than that of TDF. Tenofovir alafenamide fumarate (TAF) is currently approved by the USFDA as an active ingredient in combination with additional ARVs for treating HIV infection in the pharmaceutical products GENVOYA®, ODEFSEY® and DESCOVY®.
While each of the foregoing drugs is effective in treating HIV infection and AIDS, there remains a need to develop additional HIV antiviral drugs including additional RT inhibitors. A particular problem is the development of mutant HIV strains that are resistant to the known inhibitors. The use of RT inhibitors to treat AIDS often leads to viruses that are less sensitive to the inhibitors. This resistance is typically the result of mutations that occur in the reverse transcriptase segment of the pol gene. The continued use of antiviral compounds to prevent HIV infection will inevitably result in the emergence of new resistant strains of HIV. Accordingly, there is a particular need for new RT inhibitors that are effective against mutant HIV strains.